Many existing navigation and other GPS-enabled devices use a ceramic patch antenna connected to a GPS receiver. This is because ceramic patch antennas offer several advantages. Firstly, provided that the ceramic patch is not too small, good right-hand circular polarization (RHCP) can be obtained. GPS radio signals are transmitted using RHCP. Generally, ceramic patch antennas larger than about 15 mm×15 mm×4 mm provide good RHCP reception. Also, the radiation pattern of a horizontally mounted ceramic patch antenna gives good coverage of the upper hemisphere when the patch is mounted horizontally at the top of a device and facing the sky. Circular polarization is also used in many other telecommunication systems, such as SDARS and DVB-SH.
Unfortunately, ceramic patch antennas also suffer from significant drawbacks. When the patch is made smaller and more commensurate with the requirements of modern consumer devices (patch sizes typically 12 mm×12 mm×4 mm or less) most of the advantages are lost. The RHCP characteristic is reduced and the polarization becomes more linear unless a large ground plane is placed under the antenna, which is not practical in a mobile or hand-held device. Also the efficiency is reduced and the radiation pattern becomes more omnidirectional, with less gain toward the sky. Furthermore, the bandwidth of the antenna becomes very narrow, making manufacturing tolerances critical and increasing the cost.
In general, ceramic patch antennas have a very high Q and cannot be fine-tuned using external matching circuits. The high Q implies a narrow bandwidth and this in turn means that in different applications the same antenna requires tuning in order to be on frequency. Because matching circuits cannot be used, the ceramic patch has to be physically changed to tune it for a specific design. This requirement for physically changing the antenna increases the cost and the length of the integration process for every new application. Essentially, a new ceramic patch design must be created for each application.
Perhaps the greatest disadvantage of the ceramic patch antenna is the severe constraint it places upon the minimum thickness of a GPS-enabled device, as the thickness must be at least 12 mm to accommodate the ceramic patch. In a typical application, such as a navigation device in a car, there is a vertically mounted flat-screen display and potentially the device could be made quite thin were it not for the need to encompass the width of the ceramic patch. Finally, ceramic patches are expensive to manufacture compared to many other types of small antenna.
FIG. 1a shows a typical GPS-enabled consumer device comprising an LCD display 1, a main PCB 2, a groundplane 3 and a ceramic patch antenna 4. FIG. 1b shows how the minimum device thickness is dictated by the antenna 4, which is mounted horizontally on top of the vertical PCB 2.
Although other types of antenna are available that can solve some the above issues, none really match the performance of a large patch for GPS applications and so where optimal performance is required, large patches continue to be used and consumer devices are made thick enough to encompass the patch.
An example of a known antenna is disclosed in US2008/0158088, in the form of a class of thin antenna for GPS applications. However, such antennas are linearly polarized (see paragraph [0009]), and therefore not comparable with modern ceramic patch antennas. A further drawback of the antennas disclosed in US2008/0158088 is that in order to feed the antenna it is necessary to solder a coaxial cable directly to the antenna structure, and the antenna cannot be fed directly by the host PCB. This also means that there is no provision for a matching circuit, so the antenna must be self-resonant at the desired frequency, and the physical structure of the antenna must be changed in order to adjust the antenna to any particular host device.
Another example of a known antenna is disclosed in US2007/0171130. Although the superficially similar to some embodiments of the present invention, there are important differences. First of all the problem to be solved is very different, as US2007/0171130 teaches how to design an elongated multi-band antenna with broadband function for cellular communications, and no importance is given to the circular polarization properties of the waves generated by the antenna and the shape of the radiation pattern, which are important for satellite communications. Furthermore, the structure defined in US2007/0171130 requires a connection using coaxial cable soldered directly to the antenna, and therefore it suffers from the same drawbacks discussed above for US2008/0158088.
A further antenna is known from EP0942488A2. In this case the antenna can generate a circular polarized wave; however, because the two arms forming the antenna are arranged in perpendicular directions, such type of antenna is not suitable for application in thin devices. The same consideration applies to the antenna type disclosed in US2008/0284661.
US20055/0057401 discloses an antenna comprising an active arm and a passive arm that are mounted over a groundplane with a slot between the two arms. However, the groundplane is much larger in area than the area under the active and passive arms, and the arms are all fed and grounded at the same end of the antenna device. This antenna is not stated to have any circular polarization properties, nor can it be formed from a single sheet of metal.
The problem to be solved is thus to create a low-cost antenna that occupies a small space, can fit inside thin flat-screen devices, requires little or no customisation when installed on many different types of platform and yet will give the performance of a ceramic patch antenna.